The present invention relates generally to resonant microstructures, and more particularly to Q-control for resonant microstructures and electronic filters using such microstructures.
The need for high-frequency bandpass filters with high selectivity for telecommunication systems has stimulated interest in integrated versions of such filters wherein entire systems may be integrated onto a single silicon chip. Examples of systems requiring these filters include radio-frequency (RF) receiver systems, mobile phone networks, and satellite communication systems.
Previously, intermediate frequency (IF) filtering in frequency modulated (FM) receivers has been performed at 10.7 Mega-Hertz (MHz) IF frequency, using highly selective inductive-capacitance (LC) ceramic or crystal filters. Recently, integrated versions using integrated circuit (IC) switched-capacitor techniques have been attempted. However, designs based upon a coupled biquad filter architectures suffer from dynamic range reduction introduced when attempting high-Q operational simulation of LC stages. (Q is a figure of merit equal to reactance divided by resistance. The Q of a system determines the rate of decay of stored energy.) Modulation filtering techniques, such as N-path designs, suffer from the generation of extraneous signals, such as image and clock components inside the signal band, resulting from the remodulation process.
Recent advances in micromachining offer another analog, high frequency, high-Q, tunable integrated filter technology that can enhance filter performance over that of previous integrated versions while maintaining design characteristics appropriate for bulk fabrication in very large-scale integrated (VLSI) systems. Specifically, micromachined mechanical resonators or resonant microstructures may be used. These microresonators are integrated electromechanical devices with frequency selectivity superior to integrated resistance-capacitance (RC) active filtering techniques. Using integrated micromechanical resonators, which have Q-factors in the tens of thousands, microelectromechanical filters with selectivity comparable to macroscopic mechanical and crystal filters may be fabricated on a chip.
Since the passband shape of these filter designs depends strongly on the Q of the constituent resonators, a precise technique for controlling resonator Q is required to optimize the filter passband. Such a Q-control technique would be most convenient and effective if the Q was controllable through a single voltage or an element value, e.g., a resistor, and if the controlled value of Q was independent of the original Q.
An object of the present invention is thus to provide feedback techniques for precise control of the Q-factor of a micromechanical resonator.
Another object of the present invention is to provide very high Q microelectromechanical filters constructed of Q-controlled microresonator biquads in biquad filter architectures. In addition, the invention provides a means for passband correction of spring-coupled or parallel micromechanical resonators via control over the Q-factor of the constituent resonators.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.